Amino acids shown to inhibit lysine uptake by brain slices in vitro will be examined individually or in combination for their effects on food intake and growth of the rat. The animals will be fed a diet that is marginal in lysine to which the amino acids have been added. At intervals after they have ingested a meal of the test diet animals will be killed and blood and brain amino acid concentrations will be measured. Relationships between brain amino acid patterns and food intake responses will be examined. Brain will also be analyzed for serotonin and norepinephrine content under various conditions in which amino acid and protein supplements have been shown to alter food intake.